1. Field of the Invention
This invention relates to ophthalmic glass compositions, and more specifically to a glass composition for use in making ophthalmic lenses or lens blanks which can subsequently be toughened by an ion-exchange process.
2. Description of the Prior Art
It is well known that a freshly formed glass article has a high strength which is rapidly reduced by the formation of minor superficial abrasions in use, which form starting points for fractures when the article is put under stress.
A process of toughening glass by ion-exchange is known, for example, from British Specifications Nos. 966,733 and 1,172,383. It consists essentially in exchanging alkali metal ions (e.g Na.sup.+ ions) in a surface layer of the glass for larger alkali metal ions (e.g K.sup.+ ions), generally from a molten salt bath, at a temperature below the strain point of the glass, so as to cause a compressive stress in the surface layer thereby reducing the susceptibility to superficial abrasions and the tendency to start fractures therefrom. As described in the Specifications mentioned above, it is important that the depth of the surface layer affected by the compressive stress, i.e. the depth of penetration of the exchange ions, should be greater than that of the superficial abrasions caused in normal use of the article, and glasses of the R.sub.2 O - ZrO.sub.2 - SiO.sub.2 type and particularly of the R.sub.2 O - ZrO.sub.2 - Al.sub.2 O.sub.3 - SiO.sub.2 type, have been found particularly useful in enabling an adequate depth of penetration to be achieved in a reasonably short time, thereby giving the article a relatively high abraded strength.
Glass compositions of the R.sub.2 O - ZnO - Al.sub.2 O.sub.3 - SiO.sub.2 type have also been found useful in this connection, as described in German Offenlegungsschriften 2,205,844 and 2,300,014.